The NAVSTAR Global Positioning System (GPS) is a global navigation satellite system (GNSS) developed by the U.S. military in the 1970's. The GPS space segment has -a nominal constellation of 24 satellites, four satellites in each of 6 orbit planes.
Originally conceived as a navigation aid for ships, the use of the system has become ubiquitous both within the military and within civilian and commercial applications. For example, many cars are outfitted with GPS navigation systems that locate the car on a displayed digital map. In commercial applications, GPS systems are used for surveying in addition to controlling vehicles such as graders during the laying of road beds or tractors on farms.
The Standard Positioning Service (SPS) signal was the original signal provided to civilian users of GPS. It is made up of an L-band carrier at 1575.42 megahertz (MHz) (referred to as the L1 carrier) modulated by a pseudorandom noise (PRN)C/A (course acquisition) code. The satellites are distinguished from each other by their unique C/A codes, which are nearly orthogonal to each other. The C/A code has a chip rate of 1.023 MHz and is repeated every millisecond. A 50 bit per second data stream is modulated with the C/A code to provide satellite ephemeris and health information. The phase of the C/A code provides a measurement of the range to the satellite. This range includes an offset due to the receiver clock and is therefore referred to as the pseudo-range.
Newer signals have been provided for civilian applications. One of these new signals is an unencrypted code located at 1227.60 MHz (referred to as the L2 carrier), and will be available for general use. The other signal, located at 1176.45 MHz (referred to as the L5 carrier), will be available on the GPS satellites scheduled for launch beginning in 2005. This new L5 signal falls in a band reserved for aeronautical radio navigation.
Other GNSS exist in addition to GPS, such as the Russian GLONASS and European GALILEO systems. Position detection systems can use one or more of these systems to generate position information.
To perform a three dimensional position fix, a GNSS position detection system traditionally requires a minimum of four satellites: one satellite phase measurement for each of the spatial dimension unknowns; and since the receiver clock error is common to all satellites, it represents an additional unknown for which a solution is required. The positioning accuracy provided by the SPS is on the order of ten meters.
Differential GPS (DGPS), or more generally, differential GNSS, is a variant method for providing higher positional accuracy. If a reference GPS receiver is placed at a known location on the ground, the bulk of the errors associated with the satellite phase measurements can be estimated. Phase corrections can be calculated and broadcast to a roving GPS user. Since most errors are highly correlated in a local area, the roving user's position solution after applying the corrections will be greatly improved.
Traditional DGPS systems use the C/A code phase measurements to arrive at position solutions. These systems provide 95% positioning accuracies on the order of a few meters. The precision of the L1 carrier phase measurement has been used to improve the performance of DGPS. Using carrier smoothed code techniques, DGPS performance improves to the meter level.
Further improvements are achieved through the use of kinematic DGPS or differential carrier phase position detection. This method refers to the use of the differentially corrected carrier phase measurements, possibly in addition to the code phase. Due to the short wavelength of the L1 carrier phase (about 19 centimeters), these measurements are extremely precise, on the order of several millimeters. Although the measurements can be corrupted slightly by error sources, the potential accuracy of kinematic positioning is on the centimeter level.
The carrier phase measurements, however, have an integer cycle ambiguity associated with them. This ambiguity arises from the fact that each cycle of the carrier phase is indistinguishable from the others. Thus, before centimeter level positioning can be achieved, the integer ambiguity must be resolved.
Some differential carrier phase position detection systems use a common clock to process carrier signal information from multiple antennas. This allows for position solutions with carrier signals from less than four satellites if the relative fractional phase delay associated with the carrier signal information from the various antennas is known or can be derived.